Add warning in PTEModel when not defined

[Jack-Khuu]

No description provided.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchchat/1468

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❌ 1 New Failure, 9 Pending, 2 Unrelated Failures

As of commit f227f2e with merge base 2b555b1 ():

NEW FAILURE - The following job has failed:

• pull / test-build-runner-et-android / linux-job (gh)
  RuntimeError: Command docker exec -t d7400d70846dbd1854a1b1d7ddff720064b81207d1525cd6662fcc3b4a044ff0 /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-tinystories-executorch (16-core-ubuntu) (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-tinystories-executorch (macos-14-xlarge) (gh) (trunk failure)
  RuntimeError: Failed to load ET compiled /Users/runner/work/torchchat/torchchat/stories15M.pte

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